1. Field of the Invention
The present invention relates to a protection circuit which protects a circuit from electrostatic breakdown, and particularly to a protection circuit using a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Description of the Related Art
A semiconductor integrated circuit includes pads which allow signals to be input/output. If static electricity or surge noise is applied to such a pad, in some cases, it has an effect on the reliability of circuit elements included in the semiconductor integrated circuit. In order to solve such a problem, a protection circuit employing a zener diode or the like is provided for the pads.
For example, in a case in which a positive voltage signal is transmitted via a pad to be protected, a protection diode is arranged in an orientation such that the cathode thereof corresponds to the pad side and the anode thereof corresponds to the ground terminal side.
[Patent Document 1]
    Japanese Patent Application Laid Open No. H11-284130[Patent Document 2]    Japanese Patent Application Laid Open No. 2000-98338
However, in some cases, an audio signal, which is supplied to speakers or headphones, swings between the positive voltage side and the negative voltage side with the ground electric potential as the center. In a case in which the aforementioned protection diode is connected to such a terminal via which an audio signal is transmitted, when the audio signal swings into the negative voltage side, the protection diode is switched to the forward state, i.e., the ON state, leading to a problem in that the signal is clamped, or a problem in that leak current occurs. That is to say, such a protection diode cannot be applied to a terminal at which the voltage can fall below a voltage smaller than the ground voltage, and thus there is a limit to the voltage range to which such a protection diode can be applied.